1. Field of the Invention
The present invention relates to a magnetoresistive effect element and a magnetoresistive random access memory.
2. Related Art
In recent years, many kinds of solid-state memories for recording information based on new principles have been suggested. Among those memories, attention is currently drawn to magnetoresistive random access memories (hereinafter referred to also as MRAMs) as solid-state magnetic memories utilizing tunneling magnetoresistive effects (hereinafter referred to also as TMR effects). The MRAMs are characterized in storing data in accordance with the magnetization state of each MTJ (Magnetic Tunnel Junction) element.
In a MRAM of the above conventional type that performs writing in a current magnetic field (a magnetic field induced by a current flowing through a wiring line), when the size of each MTJ element is reduced, the coercive force Hc becomes larger, and the current required for writing tends to become higher accordingly. In the above conventional MRAM, it is impossible to reduce the cell size while maintaining a low current so as to achieve ultrahigh capacity exceeding 256 Mbits.
MRAMs utilizing a spin momentum transfer (SMT) writing method (hereinafter referred to as the spin-injection writing method) as the writing method to counter the above problem have been suggested (see U.S. Pat. No. 6,256,223, and C. Slonczewski, “Current-driven excitation of magnetic multilayers”, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, VOLUME 159, 1996, pp. L1-L7).
In a spin-injection magnetization reverse, the current Ic required for reversing magnetization is determined by the current density Jc. Accordingly, when the device area becomes smaller, the injection current IC required for reversing magnetization through spin injection also becomes lower. In a case where writing is performed at a constant current density, the writing current becomes lower when the size of the MTJ element becomes smaller, unlike in a MRAM of the conventional current-field writing method. Therefore, excellent scalability can be expected at least in principle.
In a tunneling magnetoresistive effect element of the spin-injection magnetization reversing type, an alloy containing Fe is normally used for the magnetization free layer or the reference magnetization layer. However, due to oxidization of Fe at the interface between the barrier layer and a magnetic layer (the magnetization free layer or the reference magnetization layer), the resistance R of the MTJ element becomes higher, and the current that can be applied becomes lower in a case where only a limited bias voltage can be applied. As a result, the current required for spin-injection writing cannot be supplied.
Furthermore, the TMR ratio required at the time of reading the barrier layer information becomes lower.